1. Field of the Invention
The present invention relates to a magnetic sheet, which is capable of suppressing undesirable electromagnetic waves emitted from an electronic equipment, and inhibiting electromagnetic disorders caused due to the interference of the undesirable electromagnetic waves, and also relates to a production method thereof which is efficient and low cost.
2. Description of the Related Art
Along with developments of electronic equipments, e.g. personal computers and mobile phones, such as down sizing and higher frequency, these electronic equipments suffer from a noise interference due to outer electromagnetic waves and interference of noises occurred within the electronic equipment. In order to suppress such interferences, various countermeasures have been taken for the noises. For example, a magnetic sheet (noise depressing sheet) is disposed adjacent to a noise emitting source or a noise receiving source.
The magnetic sheet is formed by adding alloy (magnetic powder) such as Fe—Si—Al to an epoxy resin, acryl resin, or the like, and then being cured to formed into a sheet by hot pressing.
As a recent trend in the art, radio communication using a coil antenna in a electromagnetic induction method, which is represented as mobile information terminals having IC tags referred as Radio Frequency Identification (RFID), has been widely used. For example, in the mobile information terminal, various conductors (metals) such as a metal casing or metal parts are disposed adjacent to an antenna element for transmitting and receiving due to downsizing of the mobile information terminal. In this case, a magnetic field usable for the transmission is largely attenuated because of the metal present adjacent to the antenna element, and as a result, a distance for RFID transmission in an electromagnetic induction system may be shorten, or transmission or receiving of a radio frequency may become difficult because a resonance frequency is shifted. To prevent these electromagnetic disorders, a magnetic sheet is placed between the antenna element and the conductor.
However, there is a case where a thickness of the magnetic sheet is increased as a battery pack is expanded due to the repetitive charging. In this case, the magnetic sheet is adhered to a cover of the battery pack of the mobile phone, and it is difficult to open and close the cover.
To solve this problem, there has recently been proposed a magnetic sheet in which an insulating support is bonded to a surface of a magnetic layer with an adhesive. With this magnetic sheet, the insulating support is present in between a cover of a battery pack and a magnetic layer, and thus the insulating support exhibits lubricity performance to prevent an adhesion between the cover of the battery pack and the magnetic layer when the magnetic layer is expanded with heat, and the opening and closing failure of the cover is prevented.
For this reason, many magnetic sheets having a structure in which a magnetic layer and an insulating support are bonded to each other with an adhesive (see Japanese Patent Application Laid-Open (JP-A) Nos. 2007-165701, 2007-123373, 2006-301900, and 2001-329234).
However, as these magnetic sheets use a binder or an adhesive, a magnetic sheet (a magnetic layer) needs to be thinner by the thickness of the adhesive. Moreover, when heat is accumulated in an electronic equipment, the adhesive and the like may be bleed out inside of the electronic equipment, causing the failure of the electronic equipment.
Moreover, a magnetic sheet formed by boning a magnetic layer and an insulating support with an adhesive is produced in its production process by curing by hot pressing to form a magnetic sheet, then forming an adhesive layer on the magnetic sheet, and furthermore laminating an insulating layer on the adhesive layer. Therefore, the production process is complicated and may be resulted in high production cost.
Moreover, an insulating support is scratched by opening and closing of a cover, and appearance thereof is impaired.
Therefore, it has been strongly desired to develop an easy and simple production method of a magnetic sheet at low cost without using an adhesive, and a magnetic sheet which has an excellent surface lubricity and high magnetic permeability without an adhesive layer.